1. Field of the Invention
The present invention relates to a system for producing high alternating voltages with a series resonant acceptor circuit consisting of a capacitance and an inductance, for use, preferably, in the high-voltage testing of electrical insulations.
2. Description of the Prior Art
Electrical insulations must be tested with high voltages in order to prove their insulating properties. Actually, all insulation operates like a capacitor with low dissipation factor. Usual testing of such insulations is done by means of high-voltage transformers, which means that practically their test load is merely capacitive, yielding reactive power. When testing very high capacitances, for instance high-voltage cables or metalclad, gas-insulated high voltage switch systems, the nominal power of the transformers must be high. Such types of test transformers are expensive and heavy and, in addition, must be fed by a variable high-voltage source, properly dimensioned for the nominal power of these transformers, unless the capacitive idle power is compensated by chokes. This compensation of the idle power may be achieved in two possible ways. If the capacitive current of the insulation to be tested is compensated at the high voltage side of the test transformer by switching a high-voltage choke in parallel to this insulation, the test transformer and the variable voltage-source are relieved. With the second method, a choke for a correspondingly lower voltage is switched parallel to the primary side of the test transformer, so that only the variable voltage source--usually a variable ratio transformer or a.c. generator--is being relieved. Both of these methods are expensive, however, and as a rule are not economical.
It is possible also to compensate the capacitive load by oscillatory circuits. A pamphlet issued by the name of "Hipotronics" (USA, undated), describes high-voltage testing systems with a parallel or series-resonant circuit. These systems are fed by voltage sources with a fixed frequency so that in order to obtain high voltages (in a series-resonant circuit) or a complete or extensive compensation, respectively, of the idle power (in a parallel resonance circuit) continuously adjustable chokes are of necessity directly designed as high-voltage chokes. In "High Voltage Engineering" (E. Kuffel and M. Abdullah, Pergamon Press, 1970, p. 100 et seq.) an embodiment is described in which a continuously variable inductor for low voltages works together with a high voltage transformer, which must be properly dimensioned for the full capacitive load of the insulation to be tested. Thus, in both cases the net feeding power supply is completely relieved of idle power; a variable voltage source, however, is still necessary for the adjustment of the desired voltage, also covering the losses of the oscillatory circuits.
As a rule, the alternating voltage tests of insulations are performed with technical frequency, i.e. both 50 Hz or 60 Hz, for which the commonly known, above mentioned, test transformers as well as the equally aforementioned oscillatory circuits are suited. If different frequencies are to be utilized, however, they will have to be produced by electric machines (generators) which are able to create the sine-waves determined for testing transformers in accordance with international rules for voltages with the necessary power. Inasmuch as the voltages of these generators also must be continuously adjusted over wide areas, they become expensive and complicated. If such generators are utilized to feed the above mentioned oscillatory circuits, they will have to be technically still more complicated, since the frequency (r.p.m.) of the generators will have to be extremely stable in order to maintain the oscillating frequency, and for the voltage to remain stable at the adjusted level. The production of high alternating test voltages with a frequency different from that of the line frequency, by means of the known methods, therefore, is very demanding.
The application of an increased voltage test by means of technical frequency of 50 Hz or 60 Hz, respectively, in winding tests of inductors, power- or high-voltage transformers is impossible wherever a ferromagnetic circuit is present which may be saturated. Furthermore, it is often advisable and necessary to test the electrical insulations with a frequency which is less than, or sometimes also higher than, the line frequency, in order to examine insulating properties in dependence from the frequency. Thus, for instance, aging processes in solid insulating materials which are caused by use in connection with electricity, may be highly advanced by relatively high frequencies (2 kHz), particularly wherever so-called partial discharges take place within the insulating substance, for instance in cavities.